MOSCOW, July 12 (Reuters) - Dmitry Medvedev, the deputy secretary of Russia's powerful Security Council chaired by President Vladimir Putin, said late on Tuesday that the increase in military assistance to Ukraine by the NATO alliance brings World War Three closer.

Commenting on the first day of the summit of the U.S.-led alliance in Lithuania, where a number of countries pledged more weaponry and financial support, Medvedev said the aid would not deter Russia from achieving its goals in Ukraine.

"The completely crazy West could not come up with anything else ... In fact, it's a dead end. World War Three is getting closer," Medvedev wrote on the Telegram messaging app.

"What does all this mean for us? Everything is obvious. The special military operation will continue with the same goals."

Russia calls its actions in Ukraine a "special military operation," while Kyiv and its allies say Moscow is waging an unprovoked war to grab land and dominate its neighbour.

The West says it wants to help Ukraine win the war, and Western powers have already supplied large amounts of modern arms and ammunition to Kyiv.

Medvedev, who cast himself as a liberal moderniser when he was president from 2008-2012, now presents himself as a fiercely anti-Western Kremlin hawk. Diplomats say his views give an indication of thinking at the top levels of the Kremlin elite.

He also advocated on Tuesday for using the "inhuman weapon" that is cluster munitions after what he said were reports of Ukraine already using it.

The U.S. announced it would supply Kyiv with cluster munitions that typically release large numbers of small bomblets over a wide area and are banned by many countries.

Russian Defence Minister Sergei Shoigu said that Moscow would be forced to use "similar" weapons if the United States supplied cluster bombs to Ukraine.

Russia and Ukraine have previously accused each other of already using cluster munitions in the 500-day war.